Composite ceramics sintered body

ABSTRACT

The strength of a composite sintered body including yttrium oxide is improved. A composite ceramics sintered body includes a matrix of yttrium oxide and silicon carbide particles dispersed within the matrix. A compound oxide phase including yttrium and silicon is present at the surface of the sintered body. A sintered body is obtained by compression-molding mixed powder including yttrium oxide powder and silicon carbide powder in an inert gas atmosphere of at least 1550° C. The sintered body is subjected to a heat treatment for at least 0.5 hour and not more than 12 hours in an atmosphere including oxygen gas in the range of at least 900° C. and less than 1200° C.

This is a Continuation-in-part of U.S. Ser. No. 08/002,335, Jan. 8, 1993 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a composite ceramics sintered body. A method of manufacturing such a sintered body is also disclosed. More particularly, it relates to a composite ceramics sintered body containing yttrium oxide (Y₂ O₃) having heat resistance and high strength to be employed as structural ceramics.

BACKGROUND INFORMATION

Yttrium oxide, which is a compound having a high melting point, has an excellent stability under a high temperature. Such yttrium oxide is potentially useful as a high temperature structural material or as translucent ceramics material.

However, yttrium oxide has not yet been put into practical usage as a structural material, due to its inferior mechanical properties as compared with currently available oxide ceramics such as aluminum oxide (Al₂ O₃) or zirconium oxide (ZrO₂). Various approaches have been made to improve its strength by adding an additive to a sintered body mainly composed of yttrium oxide. According to a study made by the inventors of the present application, it has been recognized that the strength of a sintered body having a matrix of yttrium oxide can be improved by dispersing silicon carbide (SiC) therein. The result of this experiment has been reported by the inventors in Fourth Autumn Symposium, Ceramics Society of Japan, 1991.

However, when silicon carbide is simply dispersed in an yttrium oxide matrix, the flexural strength (fracture strength) of the resulting sintered body is at the level of 300 MPa at the utmost. This level is not sufficient for practical usage of a sintered body mainly composed of yttrium oxide available as structural ceramics.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the mechanical strength of a ceramics sintered body including yttrium oxide as a matrix.

The present invention is directed to a composite ceramics sintered body including a matrix of yttrium oxide and silicon carbide particles dispersed within the matrix, and is characterized in that a compound oxide phase including yttrium and silicon is present at the surface of the sintered body, preferably essentially only at the surface.

According to a method of manufacturing a composite ceramics sintered body of the present invention, a mixed powder including yttrium oxide powder and silicon carbide powder is prepared. The mixed powder is compression-molded in an inert gas atmosphere at a temperature of at least 1550° C. to obtain a sintered body. This sintered body is subjected to a heat treatment in an atmosphere including oxygen gas at a temperature of at least 900° C. and less than 1200° C. for a duration of at least 0.5 hour and not more than 12 hours.

The composite ceramics sintered body according to the present invention contains at least yttrium oxide (Y₂ O₃) and silicon carbide (SiC). In particular, the present ceramics sintered body has a structure obtained by dispersing silicon carbide powder in an yttrium oxide powder. The sintered body according to the present invention is heat-treated in the atmosphere or an atmosphere containing oxygen gas, to be improved in flexural strength. Although not yet sufficiently clarified, the mechanism of such improvement in flexural strength attained by the heat treatment could be attributed to phenomena such as that the silicon carbide contained in the sintered body is oxidized and volume-expanded to cause compressive stress on the surface of the sintered body, or a compound oxide containing yttrium and silicon, for example Y₂ SiO₅, is precipitated on the surface of the sintered body to cause residual stress. This compound oxide phase exists around the silicon carbide particles. It is conceivable that the flexural strength of the sintered body is improved by the heat treatment in the atmosphere containing oxygen due to an action effected by the combination of the yttrium oxide that diffuses oxygen and the oxidized silicon carbide.

According to the present invention, a composite ceramics sintered body having a strength higher than that of a conventional ceramics sintered body containing yttrium oxide and silicon carbide is provided that can be used as a structural material.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1A shows schematically a structure of the surface of a composite ceramics sintered body of the present invention.

FIG. 1B schematically shows an internal structure of a composite ceramics sintered body of the present invention.

FIG. 2 is a graph showing the relationship between the amount of silicon carbide contained in the composite ceramics sintered body of the present invention and flexural strength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, silicon carbide (SiC) particles 2 are scattered within an yttrium oxide (Y₂ O₃) particle 1 or at the grain boundary of the yttrium oxide (Y₂ O₃) particles 1 at the surface portion of a ceramics sintered body of the present invention. A compound oxide phase 3 including yttrium and silicon exists around each silicon carbide particle 2. Referring to FIG. 1B, silicon carbide particles 2 are scattered within the yttrium oxide particle 1 or at the grain boundary of the yttrium oxide particles 1 inside the sintered body of the present invention.

EXAMPLE 1

Yttrium oxide powder of 0.4 μm in mean particle diameter and silicon carbide powder of 0.3 μm in mean particle diameter were wet-blended at a prescribed ratio in butanol for 50 hours with a ball mill (Step A, Table 1). Then the mixed powder was dried and subjected to hot press sintering in an argon flow at a temperature of 1550° C. to 1700° C. for one hour (Step B, Table 1). Test pieces were prepared from the obtained sintered body to be subjected to a flexural test according to JIS R 1601.

The test pieces were subjected to three-point flexural tests at room temperature. The flexural tests were carried out on test pieces following the sintering (Step B, Table 1). Further test pieces were subjected following the sintering to a heat treatment in the atmosphere at a temperature of 900° C. for 30 minutes and then tested (Step C, Table 1). The flexural tests were carried out on 15 test pieces respectively to obtain an average strength.

The measured results of the flexural tests are shown in Table 1 and FIG. 2. Referring to FIG. 2, black circles show the flexural strength (fracture strength) measured on the sintered test pieces. Black triangles show the flexural strength (fracture strength) measured on sintered and heat treated test pieces. It can be seen from FIG. 2 that the fracture strength is improved by approximately 30% by virtue of the heat treatment.

                                      TABLE 1                                      __________________________________________________________________________                    Step b                   Step c                                      Step a    Sintering      Relative       Strength                          Sample                                                                              Y.sub.2 O.sub.3                                                                     SiC  Temperature                                                                           Applied Density                                                                             Strength                                                                            Strength                                                                            Improvement                       Number                                                                              (Vol. %)                                                                            (Vol. %)                                                                            (°C.)                                                                          Pressure (MPa)                                                                         (%)  (MPa)                                                                               (MPa)                                                                               (%)                               __________________________________________________________________________     *1   100   0   1400   30      99.9 150  150   0                                *2   96    4   1550   30      99.9 310  440  42                                3    95    5   1550   30      99.8 320  450  41                                4    90   10   1600   30      99.7 392  480  22                                5    85   15   1600   30      99.5 387  490  27                                6    80   20   1600   30      99.5 412  500  21                                7    70   30   1650   30      99.0 410  520  27                                *8   65   35   1650   30      98.0 408  510  24                                *9   60   40   1700   30      98.5 405  500  23                                __________________________________________________________________________      *Example for Comparison                                                  

It is apparent from Table 1 that the strength of the sintered body is improved by carrying out a heat treatment step following the sintering. This increase in strength is noticeable when the added amount of silicon carbide powder is within the range of 5-30% by volume, whereby high strength is obtained.

In the present example, the composition of the material powder of each sample was determined quantitatively as shown in Table 1. However, it was difficult to measure quantitatively the composition of the eventual sintered body. That is to say, although the main form of a compound oxide of yttrium and silicon formed on the surface of the sintered body is Y₂ SiO₅, there are other various forms existing. It is therefore difficult to identify quantitatively the containing ratio of Y₂ O₃, SiC, Y₂ SiO₅ or the like to the whole sintered body. However, the yttrium oxide powder content of the starting mixture should be at least 70% by volume.

EXAMPLE 2

Sintered bodies were obtained from sample numbers 3 and 7 of the sintered bodies prepared according to Example 1. The sintered bodies were subjected to thermal treatments under the atmosphere for time periods shown in Tables 2 and 3 at temperatures of 700° C., 800° C., 1000° C., 1100° C., 1200° C., and 1300° C., respectively. Similar to Example 1, test pieces were prepared from each heat-treated sintered body according to JIS R 1601. Three-point flexural tests were carried out on the obtained test pieces. The measured results of the flexural strength (fracture strength) are shown in Tables 2 and 3.

                  TABLE 2                                                          ______________________________________                                                Processing                                                              Number Temperature (°C.)                                                                     Duration (hr)                                                                             Strength (MPa)                                 ______________________________________                                         4      *700          0.5        390                                                   *             3.0        385                                                   *800          0.5        392                                                    900          0.1        402                                                                 0.5        480                                                                 5.0        490                                                                 10.0       484                                                                 12.0       460                                                                 13.0       390                                                                 15.0       350                                                   1000          0.5        460                                                                 15.0       370                                                   1100          0.5        462                                                   *1200         0.5        350                                                   *             1.0        353                                                   *1300         0.5        250                                                   *             1.0        228                                            ______________________________________                                          *Example for Comparison                                                  

                  TABLE 3                                                          ______________________________________                                                Processing                                                              Number Temperature (°C.)                                                                     Duration (hr)                                                                             Strength (MPa)                                 ______________________________________                                         7      *700          0.5        400                                                   *                        405                                                   *800          0.5        395                                                    900          0.1        430                                                                 0.5        520                                                                 1.0        522                                                                 5.0        523                                                                 10.0       520                                                                 12.0       510                                                                 13.0       420                                                                 15.0       400                                                   1000          0.1        440                                                                 0.5        510                                                                 15.0       405                                                   1100          0.5        500                                                   *1200         13.0       420                                                   *             0.5        405                                                                 1.0        400                                                   1300          0.5        240                                                   *             1.0        230                                            ______________________________________                                          *Example for Comparison                                                  

By carrying out the heat treatment for at least 0.5 hour and not more than 12 hours at a temperature within the range of at least 900° C. and less than 1200° C., the flexural strength was significantly improved.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A composite yttrium oxide ceramics sintered body comprising a matrix of yttrium oxide particles present in said sintered body within the range of 70 vol. % to 95 vol. % and silicon carbide particles dispersed in said yttrium oxide matrix, and a compound oxide phase including yttrium and silicon present essentially only in a surface layer of said sintered body, and wherein said silicon carbide particles plus said compound oxide phase are present as a remainder of 30 vol. % at the most.
 2. The composite yttrium oxide ceramics sintered body of claim 1, wherein said compound oxide phase comprises Y₂ SiO₅.
 3. The composite yttrium oxide ceramics sintered body of claim 1, wherein said compound oxide phase is present around said silicon carbide particles in said surface layer of said sintered body, whereby said compound oxide phase envelops said silicon carbide particles in said surface layer.
 4. The composite yttrium oxide ceramics sintered body of claim 1, wherein said yttrium oxide particles have a mean particle diameter of 0.4 μm and said silicon carbide particles have a mean particle diameter of 0.3 μm. 